Second harmonic generation in anisotropic stratified media: a generalization of the Berreman method and its application to photonic materials.

We have developed a numerical method for calculating the second-harmonic generation (SHG) generated by an anisotropic material whose optical properties present an arbitrary modulation in one dimension. The method is based on the Berreman 4 × 4 matrix formalism, which is generalized to include nonlinear optical phenomena. It can be used under oblique incidences of the input beam, and is valid even when the SHG frequency is close to photonic bands, where the usual slowly-varying-amplitude approximation breaks down. As an example of application, we have studied the SHG performance of ferroelectric and helielectric fluids. The obtained results indicate that the present procedure may contribute to improving the structural design and enlarging the variety of nonlinear optical materials for application in optical devices.

Kinetic behavior of light emission in cholesteric liquid crystal lasers: An experimental study.

Some dynamical aspects of fluorescence and lasing have been studied in a dye-doped cholesteric liquid crystal by measuring the response of the material to nanosecond optical pumping. It has been found that as the pumping energy is increased the fluorescence pulse duration decreases, reaching a minimum at the lasing threshold. Above the threshold the temporal profiles are irregular and consist of a set of narrow pulses whose measured duration is limited by the detector risetime (1 ns). The results are interpreted in terms of a recently proposed model [JETP, 118, 822 (2014)] that makes use of rate equations to account for the laser generation in cholesteric liquid crystals. The prediction of such equations for an experimental configuration appropriate for fluorescence lifetime measurements is analyzed.

Thermally induced light-scattering effects as responsible for the degradation of cholesteric liquid crystal lasers.

We have studied the degradation process of the laser emission in a cholesteric liquid crystal laser. We have found that there exists a negative correlation between the laser efficiency and the amount of light scattered by the liquid-crystal sample in the illuminated area. The growth of scattering is attributed to the appearance of small imperfections generated in the sample as a result of certain thermal processes that involve the dye molecules. The scattering implies an increase of the coefficient of distributed losses, which is the main response of the rise of the laser threshold.

Spontaneous and field-induced mesomorphism of a silyl-terminated bent-core liquid crystal as determined from second-harmonic generation and resonant X-ray scattering.

The polarity and structure of the phases of a liquid crystal constituted by thiophene-based bent-core molecules is investigated by means of optical second-harmonic generation (SHG), and resonant and conventional X-ray diffraction. The material studied is representative of a wide family of mesogens that contain silyl groups at the ends of the chains. These bulky terminal groups have been reported to give rise to smectic phases showing ferroelectric switching. However, the analysis of the SHG signal before and after application of electric fields has allowed us to establish unambiguously that the reported ferroelectricity is not intrinsic to the material but stabilized by the cell substrates once an electric field has been applied. In addition, the results obtained from resonant X-ray diffraction indicate that virgin samples have antiferroelectric undulated synclinic smectic structures.

Photoisomerizable metallomesogens and soft crystals based on orthopalladated complexes.

With the aim of obtaining light-responsive liquid-crystalline palladium complexes, six palladium complexes derived from an orthometalated imine, bearing one or two azocarboxylato bridges, have been synthesized: [Pd(2)(µ-SC(10)H(21))(µ-O(2)CAzo)(L(1,2))(2)] (7 and 8), [Pd(2)(µ-SC(10)H(21))(µ-O(2)CAzo3C(10))(L(1,2))(2)] (9 and 10), [Pd(µ-O(2)CAzo)L(1)](2) (11), and [Pd(µ-O(2)CAzo3C(10))L(1)](2) (12), in which L(1) = p-H(21)C(10)OC(6)H(3)CH═NC(6)H(4)OC(10)H(21)-p, L(2) = p-H(21)C(10)OC(6)H(3)CH═NC(14)H(29), AzoCO(2)(-) = p-(phenylazo)benzoate, and Azo3C(10)CO(2)(-) = p-(2',3',4'-tris-n-decyloxyphenylazo)benzoate. Three of them (7-9), as well as the precursor Azo3C(10)CO(2)H (3), are thermotropic liquid crystals displaying nematic and smectic A mesophases, while 10-12 have been identified by X-ray diffraction to give rise to "soft" crystal phases. Electronic spectroscopy and (1)H NMR show that all of them undergo a trans-cis isomerization of the azobenzene moiety at λ = 365 nm. The molecular structure determines the photoresponse in solution, which is faster and more stable when the trisubstituted azocarboxylate is present and the motion of the azo group is not hindered by the orthometalated imine. The photoresponse has also been observed in the condensed phases, which change from the ordered phase to the isotropic liquid upon irradiation, except for compound 10, a soft crystal in which a permanent photoalignment highly sensitive to light polarization is produced. The latter is a behavior with potential applications, rather unusual in low-molecular-weight compounds.

Supramolecular-directed chiral induction in biaryl derivatives.

A thermodynamically controlled resolution has allowed for the generation of diastereomerically enriched complexes, by chirality transfer from an enantiopure building block to a dynamically racemic biaryl derivative. A switchable sense of induction could be achieved depending on the substituents of the chiral block.

Polar structures of bent-core liquid crystals with tetrathiafulvalene units.

We report on a series of three bent-core liquid crystals containing tetrathiafulvalene units. Their characterization consisted of electro-optical observations, second-harmonic generation (SHG) analysis, and x-ray diffraction measurements. Compound I exhibits a lamellar optically isotropic phase on cooling from the isotropic liquid and undergoes a nonreversible field-induced transition to a birefringent state. The two others present two-dimensional structural periodicities, being only compound II switchable. Additionally, the electron density maps of compounds II and III have been obtained based on the analysis of the x-ray diffraction intensities. According to SHG measurements the ground state is antiferroelectric in the three compounds. A quite good SHG performance is found in compounds I and II.

Bent-core liquid-crystalline derivative of tetrathiafulvalene: photoresponsivity and deracemization.

A bent-core mesogen containing a tetrathiafulvalene group is studied. The mesophase is an optically isotropic spongelike smectic phase without optical activity. The material responds to uv light in a peculiar way, showing photoconductivity and, in some circumstances, changing its mesomorphic properties. Under an electric field, large chiral domains are segregated from an initial racemic phase within a few minutes.

[Ophthalmological findings in pediatric brain neoplasms: 58 cases]. / Manifestaciones oftalmológicas en tumores cerebrales pediátricos: 58 casos.

PURPOSES: To describe the visual manifestations of brain neoplasms, and to analyze the effect of tumor control on these. METHODS: This is a descriptive retrospective study, which includes patients under 14 years of age, suffering from different brain neoplasms in our hospital between 1996 and 2005 inclusive. RESULTS: In the group of patients with low visual acuity, 44% had organic amblyopias. In 28% of cases, the amblyopia was secondary to the strabismus/nystagmus produced by the developing tumor. Corrective treatment was successful in some cases of partial or total organic amblyopia. Ophthalmologic evaluation (including perimetry and fundoscopy) enabled detection of 3 tumor relapses. CONCLUSIONS: Visual analysis (visual acuity, perimetry, fundoscopy) should be an essential component of assessment during brain neoplasm treatment and follow-up. Corrective treatment is sometimes successful in organic amblyopias

[Protected penetrating keratoplasty: surgical technique and endothelial response]. / Queratoplastia penetrante protegida: técnica quirúrgica y respuesta del endotelio.

PURPOSE: To describe the technique and to evaluate the safety of protected penetrating keratoplasty (PPK). METHODS: A technique for penetrating keratoplasty is described. The postoperative endothelial cell counts of 17 eyes in which this operative technique was used were compared with those in 24 eyes in whom the standard operative technique for penetrating keratoplasty (PK) was used. The post-operative time periods were grouped as follows: 3-6 months, 7-12 months and >12 months. For statistical analysis, the Mann-Whitney U non-parametric test was employed. RESULTS: There was no case where tissue extrusion occurred during the procedure. The endothelial cell count was similar in both groups for the 3-6 month period (PK = 2,086, DS 566; PPK = 1,858, DS 671; p = 0.2702) and >12 months period (PK = 1,574, DS 745; PPK = 1,419, DS 810; p = 0.2882). There was a significant difference in the 7-12 month period (PK = 2,255, DS 831; PPK = 1,569, DS 623; p = 0.0397). CONCLUSIONS: The described technique of PPK may reduce the risk of per-operative complications. Damage to the endothelium is not increased compared with that seen following the standard PK procedure.

Quantitative analysis of the x-ray diffraction intensities of undulated smectic phases in bent-core liquid crystals.

X-ray diffraction diagrams of undulated smectic phases in bent-core liquid crystals have been theoretically studied. The intensities of the reflections have been obtained for different layer modulations, and a general expression has been deduced for orthogonal cells in terms of the different harmonics of the distortion. The case of sinusoidal modulation is especially simple and has been studied also in oblique cells. High-quality x-ray measurements of three compounds reported in the literature have been analyzed as examples. In all cases it has been deduced that the modulation is sinusoidal and its amplitude has been easily obtained by fitting the experimental intensities. Equatorial (h0) reflections have been also considered to obtain information about the structure of defects at the maxima and minima of the undulation.

Some aspects about the structure of the optically isotropic phase in a bent-core liquid crystal: chiral, polar, or steric origin.

We have studied a bent-core liquid crystal where two different optically isotropic phases can be induced by a strong electric field. Depending on the field treatment the phases can present optical rotation or be optically inactive. The switching dynamics of the phases is studied by means of electrooptic and optical second-harmonic generation measurements. It is found that the ground state of the phases is locally antiferroelectric. The structure of the phases is consistent with a disordered version of the kind of structures recently proposed for the smectic blue phase: layered systems with high Gaussian curvature. The origin of the smectic layer distortion is discussed. It is concluded that the direct reason for the curvature of planes is of steric nature.

Structural study of a bent-core liquid crystal showing the B1-B2 transition.

An experimental study of the B1-B2 transition is carried out in a bent-core liquid crystal. The study is essentially based on x-ray measurements as a function of temperature. The B1-B2 transition is extremely unusual and implies a deep structural change from a columnar phase to a lamellar phase. We have found that the B1 phase in our compound is similar to the so-called B1 reversed phase, with an additional splay of the polarization in the columns. On approaching the B2 phase the width of the cross section of the columns increases. The transition is clearly first order, with a large hysteresis though the enthalpy content is very small. A possible mechanism for the transition is briefly sketched.

Structure of mesogens possessing B7 textures: the case of the bent-core mesogen 8-OPIMB-NO2.

A structural investigation of the compound 8-OPIMB-NO2 is carried out. This material is a mesogen formed by bent-core molecules where the so-called B7 phase was originally proposed. X-ray data are successfully indexed in terms of a oblique two-dimensional lattice. Using additional information from optical second harmonic generation several possible structural models are proposed. The models are based on molecular packing considerations. All these structures are clearly different from those of other materials previously classified as B7. The necessity of a revised nomenclature for classifying the structures of these compounds is pointed out.

Scenario-targeted toxicity assessment through multiple endpoint bioassays in a soil posing unacceptable environmental risk according to regulatory screening values.

Lanestosa is a chronically polluted site (derelict mine) where the soil (Lanestosa (LA) soil) exceeds screening values (SVs) of regulatory policies in force (Basque Country; Europe) for Zn, Pb and Cd. A scenario-targeted toxicity assessment was carried out on the basis of a multi-endpoint bioassay approach. Acute and chronic toxicity bioassays were conducted with selected test species (Vibrio fischeri, Dictyostelium discoideum, Lactuca sativa, Raphanus sativus and Eisenia fetida) in combination with chemical analysis of soils and elutriates and with bioaccumulation studies in earthworms. Besides, the toxicity profile was compared with that of the mine runoff (RO) soil and of a fresh artificially polluted soil (LAAPS) resembling LA soil pollutant profile. Extractability studies in LA soil revealed that Pb, Zn and Cd were highly available for exchange and/or release into the environment. Indeed, Pb and Zn were accumulated in earthworms and LA soil resulted to be toxic. Soil respiration, V. fischeri, vegetative and developmental cycles of D. discoideum and survival and juvenile production of E. fetida were severely affected. These results confirmed that LA soil had unacceptable environmental risk and demanded intervention. In contrast, although Pb and Zn concentrations in RO soil revealed also unacceptable risk, both metal extractability and toxicity were much lower than in LA soil. Thus, within the polluted site, the need for intervention varied between areas that posed dissimilar risk. Besides, since LAAPS, with a high exchangeable metal fraction, was the most toxic, ageing under in situ natural conditions seemingly contributed to attenuate LA soil risk. As a whole, combining multi-endpoint bioassays with scenario-targeted analysis (including leaching and ageing) provides reliable risk assessment in soils posing unacceptable environmental risk according to SVs, which is useful to optimise the required intervention measures.

Gyrotropy problem in nonhomogeneous media: The case of short-pitch chiral-smectic-C liquid crystals and incommensurate structures.

The question about the existence of macroscopic models for gyrotropy in inhomogeneous materials is examined. We have carried out a detailed polarimetric study on a short-pitch chiral-smectic-C liquid crystal, which has been taken as an example of an inhomogeneous optical medium. In accordance to the theoretical predictions, we have found that, as a first approximation, the inhomogeneous liquid crystal can be modeled as a homogeneous uniaxial medium with huge optical activity in a direction perpendicular to the optic axis. However, some deviations have been determined in the precise values of the optical activity predicted by the theory. These discrepancies have been attributed to higher-order gyration-like effects, which are usually negligible and have not been considered in previous theoretical approaches. The agreement improves when these higher-order effects are incorporated. In the light of the above results, the theory is applied to the case of incommensurate structures. The origin of gyrotropy in these materials is clarified, and it is shown that this quantity can be described by a usual macroscopic tensor. If the incommensurate structure has an inversion center we deduce that all gyration effects rigorously vanish, contrary to the opinion maintained by several authors.

Induction of ferroelectricity in the B2 phase of a liquid crystal composed of achiral bent-core molecules.

We report the observation of a transition from the antiferroelectric B2 phase to a ferroelectric phase in a liquid crystal composed of achiral bent-core (banana-shaped) molecules. The transition is induced by an electric field of magnitude larger than the switching threshold and is not reversible, i.e., the original B2 phase does not reappear upon field removal. The transformation is accompanied by a dramatic texture change, resulting in an almost optically isotropic structure in the absence of field. The ferroelectric character assigned to the structure is based on the electro-optic behavior of the material and on previously reported dielectric measurements. A short-pitch smectic-C*-type structure is proposed for the ferroelectric phase.

Interpretation of unusual textures in the B2 phase of a liquid crystal composed of bent-core molecules.

A liquid crystal material of bent-core molecules is investigated by means of optical texture observations, x-ray measurements, and miscibility studies. While the x-ray and miscibility data point towards a B2 phase, the texture is however unusual, showing optical isotropy and segregation in two domains with opposite gyrations. It is shown that the texture can be interpreted successfully in terms of a smectic-C(A)P(A) structure in small domains with random orientations. The optical activity data are also explained semiquantitatively.

Second-harmonic generation studies in the B2 and B4 phases of a banana-shaped liquid crystal.

Second-harmonic generation (SHG) measurements have been performed in the B2 phase of the achiral banana-shaped molecule with n=12 alkoxy end chains (P-12-O-PIMB). A quantitative value of the nonlinear efficiency has been obtained from SHG curves at oblique incidences, taking into account that the signal is generated by a random orientation of different domains. In the B4 phase, circular dichroism, optical absorption and SHG studies have been carried out. It has been found that there are no simple helical arrangements giving rise to selective reflection in the visible region of the spectrum. In addition, some unusual features of the SHG behavior are pointed out. It is concluded that the phase is intrinsically inactive for the SHG process. The detected signal is due to the presence of some birefringent inclusions that are created at the B2 to B4 transition and slowly disappear while the sample is maintained within the B4 phase. A structural model for these inclusions is presented.

Electric-field-induced B1-B2 transition in bent-core mesogens.

We report experimental results on two bent-shaped mesogens showing the B1 phase. Contrary to the conventional B1 structures, we have found that our compounds display a clear electro-optic effect in this phase. Under the influence of a 100-Hz square-wave electric field of moderate amplitude, an increase of the birefringence is observed. However, no switching current is detected, indicating that the materials remain antiferroelectric. For high enough fields, a drastic change in the texture occurs. This change is a phase transition to a B2 phase, and shows the typical antiferroelectric switching of this phase. The induced B2 phase has been identified to be homochiral. Upon removal of the field, the B1 structure is recovered. Our study is based on differential scanning calorimetry, x-ray, electro-optics, polarization reversal, and dielectric measurements.